Recent developments in the oil and gas exploration and extraction industries have included the provision of expandable bore-lining tubing. One such system proposes expandable bore liner being run into a section of open hole, below a cased section of bore, such that the upper end of the liner overlaps with the lower end of the existing casing, as described in GB 2 344 606 A. The lower end of the liner is anchored in the bore, and cement slurry is circulated into the annulus between the liner and the bore wall, displaced fluid from the annulus passing through the gap between the lower end of the casing and the upper end of the unexpanded liner. The liner is run into the bore with an expansion cone or swage located at the lower end of the liner and, once the cement slurry is in place, the expansion cone is urged upwardly through the liner, by supplying hydraulic fluid at an elevated pressure behind the cone. This expands the liner to a larger inner and outer diameter, and brings the outer face of the upper end of the liner into contact with the inner face of the lower end of the casing. The cement then cures, sealing and securing the expanded liner in the bore.
There are however a number of potential difficulties associated with this proposal. Firstly, as cementation takes place prior to expansion, there is a risk that the cement will set before expansion has been initiated or completed.
Further, the expansion cone moves upwardly from the lower end of the liner, such that any expansion problems may result in the cone becoming stuck part way through the liner. Access to remedy the problem is then restricted by the presence of the cone and the smaller diameter unexpanded liner above the cone.
Circumferential expansion of the liner using a cone results in axial shrinkage of the liner. Thus, difficulties may be experienced if the liner becomes differentially stuck in the bore, that is if there is a differential pressure between the bore and a formation intersected by the bore, and this pressure differential acts on the liner to hold the liner against a portion of the bore wall. The axial shrinkage of the liner will thus be resisted between the differentially stuck portion of the liner and the anchor at the lower end of the liner. This may result in the liner breaking, or in the expansion process being curtailed with the cone only part-way through the liner.
The use of pressure to urge the cone through the liner relies upon the maintenance of pressure integrity below the cone. Connections between liner sections will be subject to expansion, and should a connection leak following expansion, the expansion process may be hindered or halted. Furthermore, a sudden failure of a connection may expose the surrounding formation to undesirable elevated pressure, potentially damaging the formation and impacting on its production capabilities. Furthermore, if the formation is fractured, there may a loss of fluid into the formation, with the associated expense and inconvenience, and potential for damage to the formation.
Furthermore, the use of hydraulic pressure to urge the cone upwardly through the liner relies upon the provision of a pressure-tight seal between the cone and the liner, and thus requires the liner to conform to tight tolerances on the liner internal diameter, wall thickness and roundness. These tolerances are much tighter than standard API specifications, and consequently make manufacture of such liner relatively expensive.
Finally, when expanding a liner overlapping an existing casing utilising a cone or swage it is only possible to expand the liner to a diameter smaller than the casing, such that any further sections of liner must be of still smaller diameter.
It is among the objectives of embodiments of the present invention to obviate or mitigate these and other disadvantages of existing liner expansion proposals.